1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically deploy numerous base stations (or other types of wireless access points) for providing wireless connectivity to mobile units (or other types of user equipment). Each base station is responsible for providing wireless connectivity to the mobile units located in a particular cell or sector served by the base station. The mobile units are handed off from one base station to another as the mobile units roam throughout the wireless communication system. From the point of view of the user, robust handover techniques are critical for supporting seamless service as the mobile unit moves around. For example, users quickly become frustrated by gaps or silences in voice communication that may be caused by latency in the handover process. For another example, users would likely switch providers if calls were frequently dropped when the user roams from one cell to another.
The basic condition for initiating a handover is that the signal strength from the candidate target base station or cell is stronger/better than the signal strength from the current serving base station or cell. However, simply handing off a mobile unit as soon as the target base station appears to have a stronger signal than the serving base station can lead to a number of problems. For example, the signal strengths near the boundaries between a serving cell and its neighbor cells are (almost by definition) nearly equal. The signal strength received by each mobile unit near a boundary is therefore approximately equal and relatively small deviations can cause the relative signal strengths to flip-flop. The strength of the signals received by a particular mobile unit may also vary rapidly due to movement of the mobile unit and/or environmental changes. Consequently, the mobile unit may be rapidly handed back and forth (a phenomenon known as ping-ponging) if the hand off is performed based only on the relative signal strength. Ping-ponging consumes valuable overhead unnecessarily, degrades the perceived call quality, and can even lead to dropped calls.
Handovers can be made more robust by using a more sophisticated handoff condition. For example, conventional handovers are performed when the signal strength from the candidate cell is better than the signal strengths from the current serving cell by a certain amount determined by a hysteresis value and offset values. Each cell uses a single value of the hysteresis, e.g., 2 dB. Each cell also maintains different values for the offset that are applied to handoffs between the cell and its neighbor cells. For example, the offset value for handoffs between a serving cell and a first neighbor cell may be 1 dB and the offset value for handoffs between the serving cell and a second neighbor cell may be 2 dB. A time-to-trigger (TTT) is used to delay the hand off until the “better” conditions on the target cell persist for at least the TTT duration. In 3G technologies, the hysteresis, offset values, and, TTT are set to one golden set that is applied to all cells. The golden set is selected for convenience alone and does not provide performance benefits.